1. Field of the Invention
The present invention relates to capacitive pressure transducers. More particularly, the present invention relates to a pre-stressed pressure transducer designed to withstand the application of high pressure.
2. Description of Related Art
Pressure transducers convert an applied pressure to a readily ascertainable electrical quantity, thereby facilitating the measurement and regulation of pressure in a wide variety of applications. Exemplary capacitive pressure transducers to which the pre-stressed pressure transducer of the present invention is related are disclosed in U.S. Pat. No. 4,388,668, and U.S. Pat. No. 4,227,419, both assigned to the assignee of the present invention. These exemplary pressure transducers use a flexible measuring diaphragm and at least one additional fixed disk, both of which have a conductive surface. The conductive surfaces form the plates of a capacitor. The distance between the plates varies in response to applied pressure, as the measuring diaphragm flexes, and the pressure is thereby converted into a change in capacitance. The change in capacitance is detected via external circuitry to provide an accurate measure of the applied pressure.
In the pressure transducers described above, the diaphragm and associated fixed disk are sealed together via a sealing means, typically a glass frit. This seal is essential to the operation of the device, since any leakage or exposure to the external environment will alter the relative positions of the conductive surfaces and thereby disturb the accuracy of the resultant pressure measurement. The seal also typically serves to space the surfaces apart to establish an initial capacitance value.
Pressure transducers such as those described above are relatively inexpensive and have been widely used in engine control and other applications in which there is a danger that excessive pressure will be applied to the pressure transducer. High pressure surges can come about in conventional applications, for example, through an error or failure in a system using a dual differential transducer with two pressure inputs where the pressure is inadvertently removed from one of the inputs, creating a high pressure within the device. An internal high pressure condition might also result from backfire in an engine incorporating the sensor. The high pressure may damage the sealing means, substantially reducing the accuracy of subsequent measurements. In the case of the glass frit seal, cracked glass and a damaged seal can occur because the glass frit is weak under the tension force resulting from the internal high pressure. Other sealing means suffer from similar drawbacks. The transducer seal weakness and vulnerability to failure thus represent a significant obstacle to the accurate and cost-effective use of pressure transducers within the many applications in which high pressure may be either purposely or inadvertently applied.
Under current practice, protection for the measuring diaphragm is typically incorporated into transducer designs by allowing the diaphragm to bottom out against a fixed surface in the event of overpressure. U.S. Pat. No. 4,073,191 incorporates this feature into a differential pressure transducer. See column 3, lines 35 to 39. U.S. Pat. No. 4,905,575 illustrates a transducer with a diaphragm and base plates having contoured surfaces and recesses, respectively, in order to protect the diaphragm under excessive pressure. See FIG. 1 and column 3, lines 33 to 38. However, these features do not sufficiently protect the transducer sealing means, since internal high pressure can still increase even if the diaphragm is bottomed out.
Efforts to develop protective mechanisms for the entire transducer rather than just the measuring diaphragm have led to a number of specially designed devices for use in high pressure applications. One such device, disclosed in U.S. Pat. No. 4,617,607 and assigned to the present assignee, uses a flexible stainless steel diaphragm to block external high pressure fluid from damaging the glass frit seal of the transducer. See column 2, lines 38 to 41 and column 3, lines 2 to 17. This device requires a redesign of the internal structure of the transducer, since the metal diaphragm must seal off the high pressure region and be brought into contact with the measuring diaphragm. U.S. Pat. No. 4,879,627 illustrates another specially designed transducer with overpressure protection using dual diaphragms and overpressure stops. See FIG. 1 and column 6, lines 14 to 30. Other special overpressure designs of greater complexity are found in U.S. Pat. No. 4,949,581 and U.S. Pat. No. 5,022,270.
However, no suitable mechanism exists which can provide simple and inexpensive overpressure protection for transducer sealing means regardless of the internal design features. As mentioned above, these transducers are susceptible to high pressure surges despite being used in applications that may be for the most part low pressure. Known specially designed high pressure transducers do not disclose techniques which may be easily applied to conventional transducers to substantially increase the strength of the seals and the resistance of the device to internal high pressure conditions.
As is apparent from the above, there presently is a need for a pre-stressed pressure transducer which incorporates a simple mechanism and method for increasing the strength of the transducer seals. The mechanism and method should provide a substantial increase in seal strength without altering the internal design of the transducer, and should be applicable to a wide variety of transducers used in both low and high pressure applications. Furthermore, the mechanism should complement additional transducer high pressure protection features.